This invention relates to an ultrasonic probe having an ultrasonic propagation medium for use in medical ultrasonic diagnostic systems for examination and inspection within an examining body by transmitting and receiving ultrasonic signals.
Recently, an examination or inspection method using an ultrasonic propagation medium between an examining body or a human body and an ultrasonic probe which emits and receives ultrasonic signals has been applied to the field of medical ultrasonic diagnostic systems or the like.
Such ultrasonic probes using an ultrasonic propagatio medium are respectively disclosed in Japanese Laid-open Patent Application No. 58-7231 adn at "Pages 347 to 358 of a paper for the 46th lecture of the Ultrasonic Medical Society of Japan, 1985". Referring to FIGS. 1 and 2, the conventional ultrasonic probe utilizing such ultrasonic propagation medium will be described hereinbelow.
FIG. 1 is an illustration showing a trapezoidal scanning type of the ultrasonic probe, which can obtain a wide examining region in spite of a small contact area to an examining body. In FIG. 1, numeral 101 denotes an array of transducer elements, numeral 102 denotes an acoustic matching layer provided along the curved surface of the array 101 of the transducer elements, numeral 103 denotes an ultrasonic propagation medium arranged in front of the acoustic matching layer 102. Numeral 104 denotes lead wires respectively connected to the arrayed transducer elements, numeral 105 denotes cables which connect the ultrasonic probe to a body of an ultrasonic diagnostic apparatus (not shown), numeral 106 denotes an body being examined, numeal 107 denotes a transmission ultrasonic wave, numeral 108 denotes a reception ultrasonic wave, numeral 109 denotes an imagination origin, numeral 110 denotes a center of curvature of the arrayed transducer elements, and numeral 111 denotes an examining region.
The operation of the above-mentioned conventional example will be described hereinbelow.
As is apparent from this figure, the acoustic matching layer 102 and the array 101 of the transducer elements arranged in a convexed form are in plane contact with the examining body 106 such as the human body by means of the ultrasonic propagation medium 103 provided in front of the matching layer 102. Moreover, the ultrasonic propagation medium 103 can increase the scanning angle of the ultrasonic waves, namely, enlarge the examined region. The ultrasonic waves 107 transmitted, in order, from each of the transducer elements of the array 101 are deflected in the human dbody 106, since an acoustic velocity in the ultrasonic propagation medium 103 is lower than that in the human body 106. The deflected ultrasonic waves are reflected within the body 106, and are received by the same transducer element which has emitted the waves. As is apparent from FIG. 1, in the ultrasonic probe, the examining region 111 of the ultrasonic signals in the body 106 is of a sector corresponding to a part of a circle whose center is designated at a point 109. This is because the acoustic velocity in the ultrasonic propagation medium 103 is different from that in the human body 106.
Silicon rubber or the like is used as the above-mentioned ultrasonic propagation medium 103. Silicon rubber or the like has an acoustic impedance which is close to an acoustic impedance (about 1.5 to 1.6.times.10.sup.5 g/cm.sup.2 .multidot.sec) of the humand body 106 and an acoustic velocity (about 1000 m/sec) which is slower than acoustic velocity (about 1540 m/sec) of the human body 106.
As described above, in this ultrasonic probe, the examining region 111 is enlarged, and the contact surface of the ultrasonic probe with the human body 106 becomes flat. Therefore, there are advantages that the adhesion is good and the operation is easy.
FIG. 2 is a cross-sectional view showing the other example of the conventional linear scanning type of the ultrasonic probe. In FIG. 2, numeral 201 denotes a case, numeral 202 denotes an array of transducer elements provided at the front portion of the case 201, numeral 203 denotes a backing member provided at the rear portion of the array 202 of transducer elements, numeral 204 denotes lead wires respectively connected to the arrayed transducer elements 202, and numeral 205 denotes a cable connected to a body of an ultrasonic diagnostic apparatus (not shown). Numeral 206 denotes a body being examined, numeral 207 denotes an ultrasonic propagation medium provided between the arrayed transducer elements 202 and the examined body 206. The ultrasonic propagation medium 207 comprises a flexible bag 208 made of silicon rubber or the like in which bag degassed water 209 is contained.
The operation of the above-mentioned conventional example will be described hereinbelow.
Each of the arrayed transducer elements generates ultrasonic waves in order, with pulse voltage transmitted from the body of the ultrasonic diagnostic apparatus through the cable 205 being applied. The resulting ultrasonic waves are emitted to the examined body 206 through the ultrasonic propagation medium 207. The ultrasonic waves reflected within the examined body 206 are received by the transducer element which emits the ultrasonic waves, and are changed to electrical signals. The electrical signals are sent to the body of the ultrasonic diagnostic apparatus through the cable 205, and are processed so as to display an ultrasonic image.
By providing the ultrasonic propagation medium 207 between the examined body 206 and the portion for transmitting and receiving the ultrasonic waves, it is possible that the resolving power of the ultrasonic image in the vicinity of the transmitting and receiving portion or the surface of the examined body 206 is improved. Moreover, even if the surface of the examined body 206 has irregularities, the ultrasonic propagation medium 207 can be placed in good contact with the examined body 206. Therefore, there is the advantage that it is easy to obtain the ultrasonic image.
However, in the former of the above-mentioned conventional examples, the ultrasonic attenuation coefficient of the silicon rubber used as the ultrasonic propagation medium 103 is as large as about 1.5 dB/mm at the frequency of 3.5 MHz. Moreover, as is apparent from FIG. 1, there is a difference in thickness between the center portion and both end portions of the ultrasonic propagation medium 103. Therefore, an extremely large sensitivity difference arises between the center portion and both end portions of the arrayed transducer elements due to the difference of the attenuation in silicon rubber, so that it is impossible to avoid deterioration of the ultrasonic image. As a reuslt, there is a problem that a sensitivity correcting circuit is indispensable so as to correct the sensitivity difference. On the other hand, in the latter of the above-mentioned conventional examples, the ultrasonic propagation medium 207 comprising the rubber-made bag 208 which contains the degassed water 209 is placed in contact with the examined body 206 through a gel (not shown) so as to carry out an ultrasonic diagnosis. However, since the silicon-made bag 208 has a high permeability of water, the degassed water 209 in the bag 208 vaprizes through the silicon rubber-made bag 208 as time proceeds. Therefore, each time the ultrasonic propagation medium 207 is used, the degassed water 209 must be injected in the bag 208. Moreover, since the bag 208 containing the degassed water 209 is arranged to be thin, this bag 208 is weak against physical impacts. As a result, there is a problem that the bag 208 is occasionally broken so that the degassed water 209 flows to the examined body 206.